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  • Craig Migliacco

Determining the Cause of a Liquid Line Restriction Problem!

Updated: Sep 12, 2019



First of all, a liquid line restriction problem can be confirmed by measuring the following before the evaporator coil freezes:


Low Vapor Saturated Temp, High Superheat, Normal to High Subcooling, Low Delta T


Some Causes of a Liquid Line Restriction:

· Clogged filter drier

· Partially closed liquid line service valve

· Jammed solenoid valve

· Clogged screen before and/or after the metering device

· Undersized metering device

· Clogged metering device

· Stuck TXV

· TXV that has lost bulb pressure


The following are testing methods used to determine which component is causing the liquid line restriction:


Clogged Filter Drier: A hermetically sealed filter drier used on residential systems will usually have less than .5° F temperature drop across it. In most cases on smaller filter driers, the temp drop across the drier is less than .3° F. For instance, if the temperature drop across the drier is 3.0° F, the filter drier is clogged. This is usually measured with a dual temp meter with a temp sensor on the tube on each side of the drier. This way, both temps can be measured at the same time and a differential can be displayed on the temp meter. This is usually the first component checked after determining that a liquid line restriction exists since it is the easiest to check.


Liquid Line Service Valve: This specific case would be a rare problem. For this to occur, the previous technician would have to leave the valve almost closed. This could not occur on a three-position service valve because the valve could not accidentally be left in the partially closed position. This is because the valve stem must be back-seated before disconnecting the refrigerant manifold set hose. Despite the unlikelihood of this problem occurring, it is still worth checking and here is the process.


On a two-position service valve, simply remove the top cap and visually check to see if the stem inside the valve is almost all the way up. You can also take a temperature reading on both sides of the valve to see if there is a temperature drop. The same value as the filter drier can be used for testing. You shouldn’t read higher than a .3° F temp drop. There should be no pressure drop across this and therefore there should be no temp drop other than a temp loss due to the copper and brass exchanging heat with the surrounding air.


Solenoid Valve: These are rarely seen on air conditioning systems but use the .3° F temp drop for testing across the valve. There should not be any more than .3° F temp drop across this since the body of the valve is a small heat sink. There should be no pressure drop across this and therefore there should be no temp drop other than a temp loss due to the copper and brass exchanging heat with the surrounding air. It is possible for a solenoid valve to be hung up and only partially open so always test it even if it seems to be functioning.


Screen: This is used as a protective net to catch any debris from entering the metering device when installed upstream of the metering device. When this is installed downstream of the metering device, it helps to separate the liquid refrigerant molecules in order to aid in the vaporization process. The screen can get clogged due to copper shards leftover from cutting and reaming the copper tube prior to brazing. It can also get clogged due to oxidation on the inside of the tubing reacting with rust inhibitors in the refrigerant oil which can form a sludge. Additionally, the screen could get clogged by parts of the filter drier breaking off and flowing into the screen thereby clogging it. Foreign material from outside the system could also make its way into the screen through the connection of the manifold set and hoses to the system depending upon the location of the filter drier.


Since the screen is found so close to the metering device, a restriction upstream of the metering device is usually not able to be determined by taking temperature measurements. The system should be pumped down or the refrigerant recovered in order to visually inspect the screen. This should be done after checking the filter drier, solenoid, liquid line service valve, and testing the TXV (described below).


Undersized Metering Device:

The TXV (thermostatic expansion valve) size can be determined by reading the model number stamped on the outside of the valve.


If the piston is thought to be undersized, it cannot be checked until the unit is pumped down or a full recovery is performed. Compare the number size on the piston to the manufacturers sizing chart for the system’s capacity.


Capillary tube is rarely undersized since this is installed at the factory and is engineered for the specific model unit. Rather, it is possible that the capillary tubing is partially clogged and that is the problem.


Clog in the Metering Device: A clog in the piston or capillary tube can only be checked after the system is pumped down or the refrigerant is recovered. The best method for resolving a clogged capillary tube is to just replace it. In a pinch, pressurized nitrogen or a fine wire such as a piano/guitar string could work. A clog in the piston can simply be cleaned after removing the piston from the chamber and then reinstalling the piston once cleaned.


Stuck TXV: A TXV can get stuck due to overheating the valve upon initial installation. It can also stick due to oxidation reacting with rust inhibiting compounds that are included in the system’s initial oil charge. Oxidation is created during the brazing process if nitrogen or another inert gas is not flowed through the system while brazing to remove the oxygen from inside the copper tubing. A TXV can get stuck just due to internal parts wearing. The most common reason for a TXV to get stuck is due to the loss of refrigerant from the powerhead, cap tubing, and bulb assembly.


TXV Lost the Bulb Charge: If refrigerant is lost from the powerhead and bulb assembly, it will cause the TXV to close down its internal opening at the pin to its lowest setting. If the TXV is in a more closed position, the TXV will not allow enough refrigerant into the evaporator coil. Refrigerant loss to the power head could be the result of rusting at the power head, a crack in the braze joint attaching the cap tubing and bulb to the powerhead, or a rubbing of one cap tube against another tube due to vibration.


If the bulb loses most or all of its refrigerant charge, it may not be possible to determine that the TXV is the problem and not the strainer while the system is running. However, in the case of a partial refrigerant loss to the power head, it is possible to determine if the TXV is bad.


While the refrigerant gauges are attached, place the TXV bulb in hot water. If the superheat decreases and the system operates effectively, you will know with certainty that the TXV is the problem. A stuck TXV may have the same result when placing the bulb in hot water so use your judgement to determine if you need to add an additive to the system to unclog the TXV or if the TXV needs to be replaced. Typically, newer systems may have a stuck TXV while older systems may have lost bulb pressure. If the superheat does not change when placing the bulb in hot water, either the bulb has fully lost its charge, or the liquid line restriction is at another location.


The Good News!


The good news about a liquid line restriction is that you, as the technician, can solve the problem on an air conditioner by replacing the filter drier, the screen (if needed) and the TXV.


In the case of a fixed orifice system, plan on replacing the filter drier and viewing the screen and the piston (clean and replace as necessary). Remember to check that liquid line service valve before getting involved with recovering refrigerant from the system and opening the lines up! Good luck out there and always think it through before diving in!


Check out our book “Refrigerant Charging and Service Procedures for Air Conditioning”.

The full outline is available at https://www.acservicetech.com/the-book


If you have already purchased our book, be sure to tell local HVACR Instructors about our book and what you think of it. We would love to get the book into the hands of the next generation of HVACR Technicians!


Published: 9/3/2019


Author: Craig Migliaccio

About the Author:

Craig is the owner of AC Service Tech LLC and the Author of the book “Refrigerant Charging and Service Procedures for Air Conditioning”. Craig is a licensed Teacher of HVACR, Sheet Metal, and Building Maintenance in the State of New Jersey of the USA. He is also an HVACR Contracting Business owner of 15 years and holds an NJ HVACR Master License. Craig creates educational HVACR articles and videos which are posted at https://www.acservicetech.com & https://www.youtube.com/acservicetechchannel